Process of insulating wire with polytetrafluoroethylene



April 23, '1957. R. w. FINHOLT Er AL 2,789,926

PROCESS oF IINSULAT'ING WIRE.L wrm POLYTETRAFLUCROETHYLENE 2 sheetssheet1 I'Filed'uamn 22, 195,5 A 1 April 23, 1957 R. w. FlNHoLT Er AL2,789,926

.PROCESS OFYINSULATING WIRE WITH POLYTETRAFLUOROETHYLENE Filed Maron 22,1955 -2 sheets-sheet 2 y- WMM United tates haten.

PROCESS F NSULATHNG WERE WKTH PLYTETRAFLURUETHYLENE Robert W. Finholt,Erie, and William Si. Wunch, Wesleyvilie, Pa., assignors to GeneralElectric Company, a corporation of New York Application March 22, 1955,Serial No. 495,944

13 Claims. (Cl. 117-213) This invention relates to wire provided with anelectrically insulating coating of solid polytetrauoroethylene and to aprocess of producing such insulated wire, and more particularly to aprocess of producing a tough llexible coating of this insulatingmaterial on electrically conducting wire in which the material isinitially pressed on the wire in powdered form, 'as by means of calenderrollers. Such insulated wire is especially useful in armature coils forlarge electric motors.

The use `of calender rollers for applying coatings of various insulatingmaterials, such as rubber, to wire is well-known in the art and theprocess is known as a calendering process. This process has theadvantage of high speed and low cost. In accordance with this process,the wire is passed downward between grooved calender rollers andpowdered insulating material is fed to the rollers so as to be pressedonto the wire as it passes between the rollers, which are suitablydriven in a direction to follow the wire as it passes between them.

This calendering process was also proposed prior to our invention foruse in applying a coating of solid polytetrauoroethylene insulatingmaterial to wire, the material being fed to the rollers in powdered formand pressed on the wire by the rollers. Thereafter the pressed coatedwire was heated to sinter or fuse the material thereby to form a tough,llexible coating.

However, prior to our invention, the coating of solidpolytetrafluoroethylene formed on wire by the calendering process wasunreliable and unpredictable both physically and electrically, and wascharacterized by persistent flaws, the sources of which were unknown. Asa result, this insulated electrically conducting wire was of suchinferior quality because of the low dielectric strength of itsinsulating coating that it was unsuitable for use in commercial electricapparatus. Its average dielectric strength was approximately 180 voltsper .001" of insulation, with low dielectric strength in the regions ofthe numerous flaws. In addition such insulated wire was unsatisfactoryfrom the physical standpoint because of the inability of the insulatingcoating to withstand the high crushing pressures existing in someelectric apparatus. More specifically, it was found that the insulatingcoating completely failed, i. e., collapsed, when subjected to acrushing pressure of approximately 3000 p. s. i. with resulting failureof its dielectric Strength, whereas in certain electric apparatus, suchas the armature coil conductors of large7 high-speed electric motors,the crushing pressure applied to the insulated wire is substantially asgreat and sometimes greater than this value of 3000 p. s. i., so that noadequate margin of safety was provided.

Therefore, an object of our invention is an electricaliy insulated wireprovided with an insulating coating of polytetrafluoroethylene having ahigh crush pressure strength.

Another object is a simple and inexpensive process for producingelectric wire on an insulation of solid polytetralluoroethylene whichhas a uniform dielectric strength.

A further object is a simple and inexpensive calendering process forproducing on electrically conducting wire e ig@ an insulating coating ofsolid polytetrauoroethylene which is uniform and strong physically andelectrically, and suitable for use in high-power, high-speed motors, aswell as in other electric apparatus.

We have found that the successful production of a uniform high qualityinsulating coating of solid polytetrafluoroethylene on wire by thecalendering process is dependent upon certain critical factors,including temperature of the powdered material, size of the particles ofpowdered material, calender roller diameter, surface conditions of thewire and the rollers, and the maintenance of the powdered materialsubstantially free from :static electric charges. In addition, we havefound that the crush strength of the insulating coating can be increasedto a satisfactory safety operating value by a heat tempering treatmentfollowing the sintering heat treatment.

Briefly, in carrying out our invention we use a finepolytetrafluoroethylene powder having a particle diameter size of .005"to .015, which powder is maintained during application to the wire bythe calendering process at a temperature of approximately F. We utilizegrooved calender rollers having a diameter equal approximately to 2.5plus 250 times the thickness of the insulating coating. Preferably thewire is polished or lubricated and the rollers machined to have asurface roughness of 30-40 micro-inches. Means are provided forpreventing static electric charges on the powdered material. After asintering heat treatment to form a tough, flexible coating, the coatedwire is heated to increase its crush strength at a temperature ofapproximately 300 C. for a period of three hours, which heat treatmentwe have found substantially doubles the crush strength of the insulatingcoating.

Further objects and advantages of this invention will become apparentand this invention will be better understood from the followingdescription taken in connection with the accompanying drawings. Thefeatures of novelty which characterize this invention will be pointedout with particularity in the claims annexed to and forming part of thespecification.

Fig. 1 shows an expanded perspective view of calendering apparatus forapplying the powdered material to two rectangular wires in parallel;

Fig. 2 is an enlarged fragmentary plan view of the rollers of Fig. 1showing the shape of the grooves for a rectangular wire;

Fig. 3 is an enlarged fragmentary view similar to Fig. 2 but showinggrooves shaped for coating round wire, four grooves in each roller forcoating four wires in parallel being shown;

Fig. 4 is an enlarged fragmentary cross-sectional view of the staticeliminators;

Fig. 5 :shows curves of the rate of feed of the powdered material andthe dielectric strength of the coating plotted against temperature indegrees F. as abscissa; and

Fig. 6 is a curve showing the variation of roller size with thethickness of the coating.

Referring to Fig. 1, we have shown a motor 1 attached by a chain 2 to asprocket 3 which is connected to drive one of the calender rollers 4.The roller 4 is supported on the stationary bearings 5. A similar roller6 is mounted on similar bearings '7 in such a position that the groovesin the peripheries of the two rollers mate to form two orifices 8a and3b between them (Fig. 2) of a suliicient size and suitable shape toaccommodate the two rectangular wires 8 and the coating. Fig. 3 shows afragmentary view of grooved calender rollers for coating four roundwires. For our purposes the term calender rollers is defined as matingrollers having peripheral grooves which define an orifice between therollers. In order to synchronize the calender rollers 4 and 6, theyafsaeae are drivingly connected by the attached gear members 9 and 10.In this process, the calender rollers d and 6 are thrust vapart by thereaction to the compression of the insulating material. This thrustforce may exceed a ton, so that calender rollers 4 and 6 must be mountedrigidly to prevent ruinous deflections.

The wires 8 to be coated are pulled downwardly by suitable means (notshown) between these rollers d and 6 through the orifices 8a and 8b andthen through a suit- Aable oven 11 for sintering the coating. The supplyof polytetraiiuoroethylene powder 12 is positioned over the rollers in ahopper 13. The polymer powder 12 passes through the chute 14 and thesieve 15 into -a vibrating inclined trough 16 which feeds the powder tothe rollers 4 and 6 in the region of the formed orifices 3a and Sb. Itshould be noted that the grooves are so placed as to provide orificesthat are balanced to prevent the rollers from warping in use. The powderis kept in the region of the orifices by an open box formed by thecalender rollers 4 and 6 and the retaining plates 17 and 18 (Figs. l, 2and 3).

The variable speed motor 19 drives an agitator or paddle 20 which causesthe polytetrauoroethylene powder to flow to the vibrating trough 16. Thetrough 16 is shown as being mounted on an electromagnetic vibratoryplatform 22. After the supply of powder reaches the calender rollers dand 6, it is compacted on the wires by the rollers under great pressure,100 to 10,000 p. s. i. with about 2,000 p. s. i. preferred, whereby theparticles of the powder cohere to each other to form a solid but brittlecoating on the wire. The motor 1 drives the rollers at a synchronizedslightly slower peripheral speed than the speed of the wires S beingcoated. Obviously, this may be a continuous process which is limitedonly by the length of the wire used. It will be understood that a`suitable tension is applied to the wires at some point below the oven11, as by winding the wires on a power driven reel, so as to draw thewires through the orifices between the rollers. The speed of the wire islimited by the heating capacity of the oven 11 and by the ability t-ofeed the polytetrafluoroethylene powder 12 to the rollers. The thicknessof the coating maybe controlled within narrow limits by adding orremoving shims 23 and 24 between the bearing holders 25, 26 and 27, and2S respectively to vary the spacing of the calender rollers 4 and 6. Weprefer that the powder retaining plates 1'7 and i8 -be secured to thebearing holders 26 and 28 only to facilitate this adjustment. To changethe thickness of the coating more than a few thousandths of an inch, thecalender rollers 4 and 6 should be replaced by `other rollers having a'suitably different diameter as pointed out hereinafter.

In order that the rectangular wires 3 are presented to the calenderrollers in the proper alignment with respect to the orifices Sa and 8b,the wires are passed through the wire guide 29 before passing throughthe orices 8a and 8b defined by the peripheiies of the calender rollers.The wire guide 29 is provided with a reservoir 29a to store a suitablelubricant as hereinafter defined.

The static eliininat'ors 30, 31, 32 and 33 are positioned to remove anystatic charge from the powder 12 and maintain it free from such charges.Any conventional static eliminators may be used, but the particular onesshown are high voltage discharge-corona type, shown in detail in Fig. 4,wherein a plurality of needles 3dextend respectively to the plurality oforifices 35 in each of the cylindrical members 32 and 33. A voltage isapplied between the needle members 34 of the two static eliminato'rs tocause the electric discharge of any charged powder particles flowing:between these lstatic eliminatiors. if the static eliminatol's are notused, a series of static discharges takes place between the wires andthe calender lrollers d and 6 and uneven ow of powder 12 results. Theunevenvow causes small physical faults vwith consequent dielectricfaults.

When practicing this process with powdered polytetraiiuoroethylene, wehave found that the electrical properties of the insulating coating areerratic and unsatisfactory unless the temperatures of the powder androllers are maintained at approximately 75 F. In Fig. 5 a shadedtemperature range i's shown wherein this process can be carried out withsatisfactory results. The line AB shows the rate of feed of the powderedpolymer with the top plateau at the left being maximum capacity of themachine, the line CD shows the average quality, i. e. dieiectricstrength, of the coating and the line EF shows the dielectric strengthminimum. This process provides a coating having uniform dielectriccharacteristics only when carried out within the rather narrowtemperature Vlimits from 68 F. to 84 F. with the best result at about 75F. This is remarkable since polytetrafluoroethylene when once applied tothe wire is satisfactory in insulation service over a temperature rangeof from approximately C. to 280 C. Prior to this discovery of thecritical temperature, the process gave unreliable results because ofwide variations in room temperature and hence powder temperature,producing an insulation that was unuseable.

lt is preferred to have the machine shown in Fig. l enclosed in an airconditioned room maintained at a temperature of approximately 75 F. Itwas found that below 68 F. the coating quality was unuseable. At lowertemperatures the feed of the powdered polytetraiuoroethylene was easilycontrolled, but the powder was so dry that it would not coheresufficiently to produce a good compacted coating. Numerous physicalimperfections causing dielectric flaws occur in the coating with theprocess carried out at a temperature below 68 F.

Also, it was found that above 80 F. the powder is sticky, the particlesadhering together so as to produce an uneven feed of the powder to thecalender rollers. Above F. this problem appears to be insurmountable.

Since this critical temperature discovery, we have learned that solidpolytetraiiuoroethylene goes through a crystalline phase change withchange in its temperature and above 84 F. is in one crystalline phasewhich is sticky and below 68 F. is in another crystalline phase which isvery dry. For the best results the process should be carried out in thetemperature region between 68 F. gto 84 F. where the powder is a mixtureof these two crystalline phases.

Referring to Fig. 6, it was found that the calender roller overalldiameter should be a function of the coating thickness desired. Thisfunction shown in the curve of Fig. 6 is very nearly represented by theformula X=250Y12.5, where X is the abscis'sa measured in inches of theover-all roller diameter and Y is the ordinate measured in inches of thecoating thickness. This formula was derived empirically when it wasfound that rollers of 5 diameter provided the best coating for athickness of .01" and rollers of l2 diameter resulting in the bestcoating at .04. It was also found that roller diameters of 8 produce thebest results for an insulation thickness of .024 and roller diameter ofl0" produced the best results for thicknesses of .028". The use ofrollers that are too small causes too small a flow of thepolytetraiiuoroethylene, leaving bare wire spots or low density spots inthe coating which are weak dielectrically. Rollers that are too largetend to pack the powder undertoo high a pressure and cause slip planeflaws in the coating which also cause dielectric flaws.

The size of the polytetrafluoroethylene powder particles used in thisprocess is very important. A powder size TF-l marketed under theregistered trademark of Teion by E. I. du Pont de Nemours and Company ismuch too large to work satisfactorily in this process. When this powderwas divided into a finer powder marketed as Teiion TF-S, it is moreuseful but we have found that TF-5 is too large. 4A special powderground nely in liquid nitrogen produces a much better coating arsasas 1.

in -this process. The approximate diameter of these powder particles isfor TF-1, approximately .04"; TF-5, .03; and liquid nitrogen ground.01". A powder size that is used in enamels is .001 diameter or less,but this very tine enamel particle size does not operate assatisfactorily in this process. The region of best operation forparticle size has been found to be between .005" and .015 particlediameter. Larger particles produce a granular insulation that is weak inits dielectric properties, While the enamel particle size is so linethat as to be difficult to feed to the calender rollers.

The relative roughness of the calender roller and wire is important. Ingeneral it is desirable that one be rough and the other be smooth. Aftermuch experimentation we believe that it is best to have smooth wires andrough rollers. We use wires which have been polished by very line emeryor crocus cloth and calender rollers which havel been machine ground.Applying a thin film of a lubricant such as a silicone oil to the wirefrom the reservoir 29a will produce the same results as polishing. Oftenthe rollers have their grooved mating surfaces prepared by a machiningprocess which leaves a surface roughness of about 30 to 40 micro inches.Using these unpolished rollers will usually be satisfactory until theybecome polished by use. This roughness on one tends to pull the powderinto the compacting region and smoothness on the other allows the How ofthe polytetratiuoroethylene powder under pressure to even out the powderdistribution and give coatings of uniform thickness on the wire whichcoatings are essential to have a flawless insulation coating on thewire.

After the Wire is coated, it is passed through the oven where thecoating is heated to a fusing temperature above 327 C., the fusingtemperature of the polytetrafluoroethylene. An oven temperature of 400C. has been found to be satisfactory, but temperatures between'350 C.and 500 C. may be used. Solid polytetrafluoroethylene forms a gel above327 C. but has no liquid state. In the fusion oven the decompositionrate is a function of temperature which doubles about every 12 C. over327 C. Thus, the temperature and the fusing time are dependent. Most ofthe time in the oven is spent in bringing the copper and the solidpolytetrafluoroethylene up to the temperature above 327 C. After thefusing, the coated wires may be separated by means such as rotary knives(not shown) or other cutters (not shown) and allowed to cool. Withoutthe fusing or sintering step, the particles do not cohere sufficientlyand consequently the coating is brittle and has a low resistance toabrasion and bending. The sintering step coalesces the particles into atough exible coating which will allow the coated wires to be twisted orbe bent to form coils.

We have found that the insulation produced by this process has amarginal mechanical crush strengthfor use in heavy duty high speedrotating electric machiney. It is not uncommon to have a pressure in therotor of a machine in excess of 2500 p. s. i. in certain regions,whereas the insulation coating produced by this calendering processoften fails, i. e., collapses completely at 3,000 p. s. i- Of course,this margin is a much too small safety factor so that the properties ofthe coated wire produced by this method as thus far described preventits use in heavy duty machines of this type. However, we have found thata three-hour tempering at approximately 300 C., as by placing the wirein an open coil in an oven maintained at this temperature, will morethan double the crush strength and give an insulation coating on thewire that will withstand crushing pressures of 6500 p. s. i. with anelastic limit of 2400 p. s. i. This insulated Wire can be bent easilyand can be bound into an armature with conventional steel wire. Thetempering to be satisfactory must be done at a temperature within thelimits of 280 C. to 327 C. The wire must not be heated to a temperatureas high as 327 C. but the closer to 327 C. it is kept the quicker theaction becomes. Practically it is risky ltoV heat the tempering oven toa temperature above 310 C. because of oven control overshoot oftemperature, and at 280 C. the process is too slow for effective use.With this added step, we are able to use the insulation for the completeinsulation of the wire to be wound into armature coils in heavy dutydynamoelectric machines with a safe operating pressure limit of 5000 p.s. i., the usual grounding' insulation in the core slots or in the endwinding not being required, nor is any outside protection of the coilsnecessary, such as with glass tape.

We have insulated wire in this continuous process at 12 ft. per minute.Any further increase beyond this speed is governed only by the abilityto feed the powder to the calender rollers, and the ability of the oven11 to fuse the coating.

ln summary, we have produced wire having a tough flexible coating ofpolytetrafluoroethylene which coating is on the order of .01 to .04" inthickness by a process of calendering. For the best results, the processshould be carried out Within the temperature range of 68 to 84 F., thepowder size be within the range of .005 to .015, the roller size beapproximately in the ratio 2.5 plus 250 times the thickness of theinsulation coating, the insulation being sintered by being heated in anoven at approximately 400 C. The crush strength of the insulation may beimproved by tempering the polymer for three hours at approximately 300C. A wire insulated in this manner provides an insulation having anaverage dielectric strength of 535 volts per .001 and a minimumdielectric strength of 430 volts per .001 of coating. The coated wirewill safely withstand a crushing pressure of 5000 p. s. i. with anelastic limit of 2400 p. s. i.

While we have shown and described the particular embodiment of thisinvention, further modifications and improvements will occur to thoseskilled in the art. For instance, the coating thickness may be more orless than that we have actually provided. We desire it to be understood,therefore, that this invention is not limited to the form shown, and weintend by the appended claims to cover all modifications which do notdepart from the true spirit and scope of this invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. The process of insulating a wire with a tough tiexible coating ofsolid polytetrauoroethylene material, which comprises pressing thematerial in powdered form on the wire while maintaining the material ata temperature within the range of 68 F. to 84 F. thereby to form acoating of compacted powdered material on the wire, and heating thecoated wire to a temperature above 327 C. to fuse the material into atough flexible coating.

2. The process of insulating wire with a tough iiexible coating of solidpolytetrauoroethylene material which comprises pressing the material inpowdered form on the wire while maintaining the material at atemperature Within the range of 68 F. to 84 F. thereby to form a coatingof compacted powdered material on the wire, heating the coated wire to atemperature above 327 C. to fuse the material into a tough flexiblecoating and thereafter heat treating the coated wire at a temperaturewithin the range of 280 C. to 327 C. to increase the crush strength ofthe coating.

3. The process of insulating a wire with a tough flexible coating ofsolid polytetrauoroethylene material by means of grooved calenderrollers which comprises passing the wire downward between the rollersand feeding the material in powdered form to the rollers whilemaintain.- ing the material wire and the rollers at a temperature ofapproximately 75 F. thereby to form a coating of com# pacted powderedmaterial on the wire, heating the coated wire to a temperature ofapproximately 400 C. to fuse the material into a tough tiexible coatingand thereafter heating the coated wire at a temperature of approximately300 C. for a period of approximately three hours to iucrease the crushstrength of the coating.

4. The process of insulating wire with a tough exible coating of solidpolytetrauoroethylene material by means of groove calender rollers whichcomprises providing the material in the form of a powder having aparticle size of .005" to .015", pressing the powdered material on thewire while maintaining the material at a temperature within the range of68 F. to 84 F. thereby to form a coating of compacted powdered materialon the wire, heating the coated wire to a temperature above 327 C. tofuse the material into a tough exible coating and thereafter heating thecoated wire at a temperature within the range of 280 C. to 327 C. toincrease the crush strength of the coating.

5. The process of insulating wire with a tough ilexble coating of solidpolytetrafluoroethylene material by means of grooved calenderrollers'which comprises providing the material in the form of a powderhaving a particle size of .005" to .015, pressing the powdered materialon the wire while maintaining the material substantially free fromstatic electricity and at a temperature within the range of 68 F. to 84F. thereby to form a coating of compacted powdered material on the wire,heating the coated wire to a temperature above 327 C. to fuse thematerial into a tough exible coating and thereafter heating the coatedwire at a temperature within the range of 280 C. to 327 C. to increasethe crush strength of the coating.

6. The process of insulating wire with a tough flexible coating ofpolytetrauoroethylene material by means of grooved calender rollerswhich comprises providing the material in the form of a powder having aparticle size of .005 to .015", providing grooved calender rollershaving a diameter in inches equal to 2.5 plus 250 times the desiredcoating thickness, positioning the rollers to give the desired coatingthickness, passing the wire between the rollers and feeding the powderedmaterial to the rollers while maintaining the material at a temperaturewithin the range of 68 F. to 84 F. thereby to form a coating ofcompacted powdered material on the wire, heating the coated wire as itleaves the rollers to a temperature above 327 C. to fuse the materialinto a tough exible coating and thereafter heating the coated wire at atemperature within the range of 280 C. to 327 C. to increase the crushstrength of the coating.

7. The process of insulating wire with a tough tlexible coating ofpolytetrauoroethylene material by means of grooved calender rollerswhich comprises providing the material in the form of a powder having aparticle size of approximately .01, providing grooved calender rollershaving roughened groove wall surfaces and having diameters in inchesequal to 2.5 plus 250 times the desired coating thickness, positioningthe rollers to give the desired coating thickness, maintaining the wire,material and rollers at a temperature of approximately 75 F., applying acoating of lubricant tothe wire, removing static electricity from thewire and the powder, passing the wire downward between the rollers andfeeding the powdered material to the rollers thereby to form a coatingof compacted powdered material on the wire, heating the coated wire asit leaves the rollers to a temperature of approximately 400 C. to fusethe material into a tough exible coating and thereafter heating thecoated wire for three hours at `a temperature of approximately 300 C. toincrease the crush strength of the coating.

8. rThe process for applying to a wire la tough flexible coating of QJolytetraiicoroethylene insulating material by means of gro-overlcalender rollers comprising the steps ot grinding the me. rial into apowder having a particle size ot appronim...ely selecting calenderingrollers having diameter in inches equal to 2.5" plus 250 times thedesired coating thickness, positioning the rollers to give the desiredcoating thickness, maintaining the wires, powder and apparatus at atemperature of approximately 75 F., applying a thin coating of-lubricant to the wire, retrieving static electricity from the wire andthe powder,

passing the wire downward between the rollers and feed-` ing thepowdered material to the rollers thereby to form a coating' of compactedpowdered material on the wire, heating the coated wire as it leavesV therollers to a temperature above 327. C. to fuse the material into a toughflexible coating and'thereafter heating the coated wire for three hoursat a temperature of 300 C. to increase the crush strength of thecoating.

9. A process for applying to a wire a tough tiexible ting of solidpolytetrauoroethylene insulating mateb" means of grooved calenderrollers comprising the steps of providing the material in powder forbearing a particle size of .005 to .015", polishing the wire to becoated, maintaining the wire, powder and apparatus to a temperaturewithin the range of 68 F. to 84 F., passing the wire to be coateddownwardly between the rollers having their periphery machine ground toa predeterlic;

mined roughness, removing static electricity from the powder, feedingthe powdered material to the rollers, heating the coated wire as itleaves said rollers to a temperature above 327 C. to fuse the materialinto a tough exible coating and thereafter heating the coated wires in atempering oven heated to a temperature within the range of 280 C. to 310C.

10. The process of insulating a wire with a tough iiexible coating ofsolid polytetrauoroethylene material,.

which comprises removing static electricity from the material and. wireto be coated, pressing the material in the form of a powder on the wirewhile maintaining the material, wire and compacting apparatus at atemperature of approximately F. thereby to form a coating of compactedmaterial on the wire, and heating the coated wire to a temperature above327 C. to fuse the material into a tough exible coating.

l1. The process of insulating wire with a tough flexible coating ofsolid polytetrauoroethylene material which comprises, lubricating thewire, pressing the material in the form of a powder on the wire whilemaintaining the materialat a temperature of approximately 75 F. therebyto form a coating of compacted material on the wire, and heating thecoated wire to a temperature above 327 C. to fuse the material into atough flexible coating.

12. The process of insulating wire with a tough flexible coating ofsolid polytetrauoroethylene material by means of grooved calenderrollers which comprises, providing the material in the form of a powderhaving a particle size of .005 to .015, pressing the powdered materialon the wire while maintaining the material substantially free lfromstatic-electricity and at a temperature of approximately 75 F. therebyto form a coating of compacted powdered material on the wire, andheating the coated wire to a temperature above 327 C. to fuse thematerial into a tough flexible coating.

13. A.- process Vfor applying to a wire a tough liexible coating ofsolid tetrauoroethylene polymer insulation by means of grooved `calenderrollers, comprising, polishing the wireto-be coated, maintaining thewire, polymer and rollers toa temperature of approximately 75 F.,passing the wire to be coated downwardly between the grooved rollershaving their grooves machine ground to a predetermined roughness,removing static electricity from the polymer in the form of a powder,conveying the polymer to the rollers, and heating the coated wire as itleaves the rollers to a temperature above 327 C. to fuse the polymerinto a tough flexible coating.

eerences Cited in the tile of this patent UNTED STATES. PATENTS2,392,338 Joyce Ian. 8, 1946 2,427,183 Berry Sept. 9, 1947 2,456,621Cheney Dec. 21, 1948 2,485,691 Bogese Oct.,25, 1949 2,538,808Y SwissIan. 23, 1951 2,547,047 Saums et al Apr. 3, 1951

13. A PROCESS FOR APPLYING TO A WIRE A TOUGH FLEXIBLE COATING OF SOLIDTETRAFLUOROETHYLENE POLYMER INSULATION BY MEANS OF GROOVED CALENDERROLLERS, COMPRISING, POLISHING THE WIRE TO BE COATED, MAINTAINING THEWIRE, POLYMER AND ROLLERS TO A TEMPERATURE OF APPROXIMATELY 75*F.,PASSING THE WIRE TO BE COATED DOWNWARDLY BETWEEN THE GROOVED ROLLERSHAVING THEIR GROOVES MACHINE GROUND TO A PREDETERMINED ROUGHNESS,REMOVING STATIC ELECTRICITY FROM THE POLYMER IN THE FORM OF A POWDER,CONVEYING THE POLYMER TO THE ROLLER, AND HEATING THE COATED WIRE AS ITLEAVES THE ROLLERS TO A TEMPERATURE ABOVE 327*C. TO FUSE THE POLYMERINTO A TOUGH FLEXIBLE COATING.